At the end of the 19th century, with continuing renovations of internal-combustion engines, the improvement on oils has also drawn wide attentions. Being an organic fuel with a low price, diesel oil has the advantages of high thermal efficiency, low oil consumption, less discharge etc., and thus, it has become a trend to utilize diesel oil in internal-combustion engines. Additionally, because the molecular weights of the alkane components in diesel oil are relatively high, the burning rate of diesel oil during the operation of internal-combustion engines is not high enough and the burning performance thereof is not good enough, which not only increases the oil consumption, but also aggravates the degree of pollution of the discharged gases to the air. Thus, it is necessary to increase the burning performance of diesel oil in engines. In recent years, oxygen-containing fuels such as methanol, methylal or the like have been added into diesel oil, which effectively decreases the discharge of carbon smoke and exhaust gas. However, these compounds have low vapor pressure and cetane number, or poor solubility with diesel oil.
In 1998, the research of David S. Moulton (U.S. Pat. No. 5,746,785) indicated that, being a novel oil additive, polyoxymethylene dialkyl ethers (RO(CH2O)nR) can improve the burning characteristic of diesel oil remarkably, increase the thermal efficiency effectively, and reduce the discharge of NOx and carbon smoke greatly. Furthermore, they have very high cetane number and oxygen content, as well as good mutual solubility with common diesel oils, and therefore are regarded as an environmentally acceptable blending component for diesel oils with a promising application prospect.
In the earlier time, polyoxymethylene dialkyl ethers are prepared by the reaction of methanol with paraformaldehyde or glycol ethylidene-formal under the catalysis of a protonic acid, wherein the reaction temperature is between 150 to 180° C. and a byproduct of CO2 is produced synchronously. In 1948, Du Pont (U.S. Pat. No. 2,449,469) investigated the acetalation reaction of polyoxymethylene ether with paraformaldehyde or concentrated formaldehyde using an inorganic acidic catalyst such as sulfuric acid under a relatively mild condition, which mainly produced polyoxymethylene dialkyl ethers wherein n=2-3.
In 2008, BASF (US 20080207954) reported a process for preparing DMM3-4 by the reaction of methanol with an aqueous formaldehyde solution using a liquid acid or a solid acid as catalyst. This reaction process included an acetalation reactor, a reactive distillator and a phase separator. An acetalation reaction was performed between methanol and an aqueous formaldehyde solution in the reactor to produce DMM1-4. Then, the crude products (DMM1-4, starting materials, and water) and the catalyst solution (the catalyst and the high boiling compounds) were separated by using a reactive distillation, and the catalyst solution was recycled for reuse. After being removed of DMM1 and a part of the reaction starting materials by distillation, the crude products then entered the phase separator for removing water by separation so as to obtain a product of DMM3-4. Taking an aqueous formaldehyde solution as the reaction starting material directly led to a low yield of product, a complex separation process and high energy consumption.
BP Company developed heterogeneous catalyst systems of borosilicate molecular sieve, sulfonic acid-based cation exchange resin or the like (U.S. Pat. Nos. 5,959,156, 6,160,174, 6,2655,284). Dimethyl ether and methanol were used as the starting materials to produce formaldehyde via a hydration reaction of dimethyl ether. Further, an acetalation reaction of formaldehyde and methanol produced DMMn. In this reaction process, the separation of the products (DMM≧2) and the reuse of the starting materials were realized in a reactive rectification manner. However, the catalyst in this method had a low activity and had to be regenerated frequently, leading to a complex process.
In recent years, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (U.S. Pat. No. 7,560,599 B2) reported a method for synthesizing DMMn by an acetalation reaction of methanol with trioxymethylene catalyzed by an ionic liquid, wherein the reaction conversion could be up to 90% and the selectivity for DMM3-8 could be up to 40%. The separation and recycling of the catalyst were realized (CN 200810150868.4).